Artificial spinal disc

ABSTRACT

An artificial disc prosthesis is provided. The prosthesis of the present invention enables spinal segment alignment by having a variable height across its surface. The variable height is achieved by an asymmetric artificial nucleus or by at least one variable height end plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of:

pending prior U.S. patent application Ser. No. 10/590,139 and entitledARTIFICIAL SPINAL DISC filed as a U.S. national stage filing of:

PCT Application No. PCT/US05/023134, filed Jun. 30, 2005 and entitledARTIFICIAL SPINAL DISC, which claims the benefit of:

prior U.S. Provisional Patent Application Ser. No. 60/658,161, filedMar. 4, 2005 and entitled ARTIFICIAL SPINAL DISC, and

prior U.S. Provisional Patent Application Ser. No. 60/584,240, filedJun. 30, 2004 and entitled ARTIFICIAL DISK FOR DEFORMITY CORRECTION.

The above-identified documents are hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to methods and devices for the treatmentof disc disease and spinal deformities with an artificial discreplacement.

BACKGROUND OF THE INVENTION

Spinal arthroplasty is an emerging field that offers the promise ofrestoring and/or maintaining normal spinal motion. The goal of spinalarthroplasty is to reduce or eliminate adjacent segment disease (ASD) bymaintaining the normal spinal biomechanics at the operative level. Toaccomplish this, an artificial cervical prosthesis must duplicate asclosely as possible the natural spinal biomechanics, includingmaintaining the axial height of the disc as well as applying angularadjustment throughout the full range of motion of the natural spine.

The spine plays an integral role in neural protection, load bearing andmotion. The vertebral column provides a strong, yet mobile central axisfor the skeleton and is composed of twenty-four vertebral bodies withseventy-five stable articulations. The intervertebral disc is afundamental component of the spinal motion segment, providing cushioningand flexibility. Adjacent vertebrae are linked together by threearticulations: a) the vertebral bodies and disc, which transmitcompressive and shear loads and provide flexibility, and b) by two facetjoints, which protect the disc from translational shear stress and limitrotation. This “triple joint complex” allows for flexion, extension,lateral bending and rotation of the spine.

The intervertebral disc is composed of an inner gel-like matrix calledthe nucleus pulposus and an outer surrounding fibrous band called theannulus fibrosus. When compressive loads are placed on the spine,increased pressure in the nucleus pulposus is transmitted to theannulus, which bulges outwards. The degenerative cascade of theintervertebral disc initially involves desiccation of the nucleuspulposus. With decreased elasticity and dampening from the nucleus,increased loads are transmitted to the annulus and facets. The increasedstress on the annulus can lead to fissures and radial tears in itscollagen fibers. With further degeneration, this can lead tocircumferential bulging of the disc, contained and uncontained discherniations, and complete desiccation of the disc. This degenerativecascade can result in axial pain, by stimulating pain fibers in theannulus, or compression of spinal nerve roots and/or the spinal cord.This can manifest itself in motor weakness, pain and/or numbness in thearms or legs or both.

The structure and function of the discs may be altered by a variety offactors including repeated stress, trauma, infection, neoplasm,deformity, segmental instability and inflammatory conditions.Degeneration of the intervertebral disc is the most common etiology ofclinical symptoms referable to the spine. Degeneration of the spine is auniversal concomitant of human aging. In the cervical spine, neck andarm pain caused by nerve root compression has been estimated to affect51% of the adult population. Spondylosis of the spine and aging areintimately related, with spondylosis increasing in both prevalence andseverity with age. Fortunately, the majority of patients will improvewithout surgery. In approximately 10-15% of cases, spondylosis isassociated with persistent nerve root and spinal cord compression and/orspinal pain, with a small percentage ultimately requiring surgery.

The most common type of surgery used in the United States for thetreatment of degenerative disorders of the spine (spondylosis) is spinalfusion. In an interbody fusion, the diseased disc is removed and eithera wedge of bone from the patient's hip, allograft or a metallic spaceris placed between the vertebrae where the disc was removed. Thisimmobilizes the functional spinal unit. While this surgery has beensuccessful in eliminating motion, there are disadvantages associatedwith it. By converting a mobile, functional spinal unit into a fixed,nonfunctional one, fusion results in increased strain patterns at levelsadjacent to the fused segment. When a segment of the spine is fused,there is elimination of motion at the level of surgery. Therefore, thestresses that would normally be absorbed by the disc at the site ofsurgery are now transferred to adjacent segments. This can causeadjacent segment disease (ASD) to one or several spinal units adjacentto the affected level. ASD can be defined as a clinical syndrome ofsymptomatic degenerative changes occurring adjacent to a previouslyfused motion segment. Retrospective studies have estimated that ASD canoccur in the cervical spine at a rate as high as 2.9% per year with aprojected survivorship rate of 26% at 10 years (Hilibrand A S, Carlson GD, Palumbo M, Jones P K, Bohlman H H: Radiculopathy and myelopathy atsegments adjacent to the site of a previous anterior cervicalarthrodesis. J Bone Joint Surg (Am) 81:519-528, 1999).

In the cervical spine, thousands of North Americans undergo surgery forcervical spondylosis each year. The majority of these procedures involvean anterior discectomy with decompression of the spinal cord and/ornerve root. The primary indication for surgery in the management ofcervical spondylosis is radiculopathy, myelopathy and/or neck pain.Following the discectomy, an anterior interbody fusion is commonlyperformed. Autologous bone harvested from the iliac crest or cadavericbone is most commonly used to fill the space created by the removal ofthe disc. A number of other solutions have been suggested, includingmetallic devices such as fusion cages or other types of spacers,xenografts such as bovine bone, and biological strategies such as theuse of growth factors. The graft for the interbody fusion can be shapedto correct underlying deformity of the cervical spine. By contouring thegraft one can restore lordosis to a straight or kyphotic spine.

A more recent alternative to spinal fusion is replacement of the damageddisc with a motion preservation device, which includes either a nucleusor total disc replacement (TDR). The rationale for the development ofthe artificial disc is to prevent adjacent segment disease. Artificialdisc devices can be broadly divided into two categories, those thatreplace the nucleus only, leaving the annulus and vertebral body endplates intact and those that involve replacement of the disc andaddition of prosthetic end plates. Both strategies are directed atrestoration of intervertebral disc function. Prosthetic nuclei aredescribed, for example, in U.S. Pat. Nos. 5,047,055 and 5,192,326.United States Patent application US2002/0183848 also discloses aprosthetic spinal disc nucleus that has a hydrogel core surrounded by aconstraining jacket.

There are several different types of prosthetic devices for use in thecervical or lumbar segments of the spine designed for TDR. For example,the Prodisc™ and the Charite™ disc are composites of cobalt chromium endplates with a polyethylene core. The Prodisc™ is described in U.S. Pat.No. 5,314,477 and the Charite™ disc is described in U.S. Pat. Nos.5,401,269 and 5,556,431. The Prestige™ disc is another type ofartificial disc that comprises a metal on metal design with a ball andtrough articulation. Another type of artificial disc that is gainingpopularity in the cervical spine is the Bryan® disc, described inseveral United States Patent applications including 2004/0098131;2004/00544411; and 2002/0 128715. The Bryan® disc is a compositeartificial disc with a low friction, wear resistant, elastic nucleusthat articulates with two circular metal plates.

Presently, there are at least four artificial cervical disc replacementsystems undergoing clinical trials worldwide. These includeunconstrained devices, such as the PCM cervical disc. Theseunconstrained devices do not have mechanical stops to limit their rangeof motion. The Bryan® Cervical disc, the Prodisc™ C and the Prestige™ LPcervical disc systems limit range of motion to varying degrees. Thesesystems can be considered semi-constrained, in that there are mechanicalstops outside the normal range of motion. Thus far, only the Charite™disc has been approved for use in the United States.

Artificial spinal discs have been implanted for the management ofdegenerative disc disease producing radiculopathy, myelopathy and/oraxial spinal pain. More recently, artificial discs have been adopted forthe treatment of trauma. The aim of TDR is to reproduce the biomechanicsof the natural disc. Early clinical and biomechanical studies withsingle and multi-level disc replacement have reported favorable clinicaloutcomes and preserved range of motion at the level of surgery.Preservation of range of motion, however, while an important feature ofan artificial disc, is only a single measure of spinal biomechanics. Theeffect of the disc on angulation at the operative level, the averagedisc space height, and overall spinal alignment (sagittal and coronalbalance) also needs to be considered.

While the introduction of artificial discs has led to many successfulsurgeries, there are still problems associated with the current discs.For example, all of the current artificial cervical discs have a fixedheight across the entire disc. The artificial discs presently availablecan have issues with focal kyphosis or kyphosis at adjacent segments ofthe spine after the patient post-operatively reassumes an uprightposition, supporting the weight of the head and body. For instance, withthe Bryan® disc, the end plates are allowed to move freely about allaxes of rotation, allowing the end plate to assume a position resultingfrom the forces exerted on the implant by the head and neck. At times,this position may be significantly different from the positioning of thedisc intra-operatively. Several published studies with the Bryan®cervical disc replacement system have reported a tendency for the endplates of the prosthesis and the alignment of the cervical spine todevelop kyphosis following surgery. [Pickett G E, Mitsis D K, Sekhon L Het al. Effects of a cervical disc prosthesis on segmental and cervicalspine alignment. Neurosurg Focus 2004; 17(E5):30-35; Johnson J P,Lauryssen C, Cambron H O, et al. Sagittal alignment and the Bryan®cervical disc. Neurosurg Focus 2004; 17(E14):1-4; Sekhon L H S. Cervicalarthroplasty in the management of spondylotic myelopathy: 18 monthresults. Neurosurg Focus 2004; 17(E8):55-61.] This kyphotic angulationof the prosthesis has been attributed .to the passive (unconstrainedmotion with a mobile nucleus and variable instantaneous axis ofrotation) design of the implant. None of the current TDR systemsaddresses this major complication.

A significant number of patients with spinal disc disease have a loss ofsagittal alignment of the spine as a result of the degenerative process.In addition, varying degrees of coronal imbalance can also occur. Noneof the available artificial disc replacement systems are designed torestore normal alignment to a spine that is straight, which havefocal/global kyphosis or coronal deformity. Existing artificial discreplacement systems that are inserted into either a straight, kyphoticor angulated segment are likely to take on the angle and localbiomechanics determined by the facets, ligaments and muscle forces. Assuch, patients with a pre-operative straight spine may developpost-operative kyphosis, and patients with a pre-operative kyphosis mayhave a worsening of the deformity post-operatively. Kyphosis of thespine has been implicated in segmental instability and the developmentof clinically significant degenerative disease. Several clinical studieshave described that a change in the sagittal or coronal balance of thespine can result in clinically significant axial spinal pain as well theinitiation and/or the acceleration of ASD. [Kawakami M, Tamaki T,Yoshida M, et al. Axial symptoms and cervical alignment after anteriorspinal fusion for patients with cervical myelopathy. J Spinal Disord1999; 12:50-60; Harrison D D, Harrison D E, Janik T J, et al. Modelingof the sagittal cervical spine as a method to discriminate hypolordosis:results of elliptical and circular modeling in 72 asymptomatic subjects,52 acute neck pain subjects, and 70 chronic neck pain subjects. Spine2004; 29:2485-2492; Katsuura A, Hukuda S, Saruhashi Y, et al. Kyphoticmalalignment after anterior cervical fusion is one of the factorspromoting the degenerative process in adjacent intervertebral levels.Eur Spine J 2001; 10:320-324; Ferch R D, Shad A, Cadoux-Hudson T A,Teddy P J. Anterior correction of cervical kyphotic deformity: effectson myelopathy, neck pain, and sagittal alignment. J Neurosurg 2004;100:S13-S19; Katsuura A, Hukuda S, Imanaka T, Miyamoto K, Kanemoto M.Anterior cervical plate used in degenerative disease can maintaincervical lordosis. J Spinal Disord 1996; 9:470-476.]

Attempting to provide a deformity correction by simply altering the endplate or the nucleus of an artificial disc, while still maintaining freemovement about all axes of rotation, may not be sustainable as theforces exerted by the head and body on the artificial disc couldcounteract the desired correction. To provide a sustainable correction,some limitation on the axes of rotation is required. From a designperspective, the goal is to design an artificial disc that is able tocorrect deformity (coronal and sagittal), has mechanical stops outsidethe normal range of motion (semi-constrained), and preferably hasvariable instantaneous axis of rotation (IAR).

The limits on the axes of rotation can fall into two categories. One isto provide correction using a permanent rotation or translation of anaxis to support the correction. This is accomplished using thegeometries of the core and end plates themselves and is referred to theGeometric Constraint category. The second is to keep free range ofmotion about all axes but provide the correction using a materialsupport. This type of design provides the correction by the impositionof a deformable material in the plane of correction for normal rotationin that plane. This is the Material Constraint category of designs.

Degenerative disc disease is a major source of morbidity in our society.It can lead to serious economic and emotional problems for thoseafflicted. Thus, there is a need for an artificial disc that canalleviate both symptoms and correct deformity (sagittal or coronal orboth) of the spine.

BRIEF SUMMARY OF THE INVENTION

There are a number of different strategies that can be used with discreplacements to address the need for alignment/deformity correction inthe spine. With most of the available discs, the angle of disc insertioncan significantly alter the orientation of the prosthesis. This isrelated to bone removal and end-plate preparation for the prosthesis. Bychanging the angle of insertion, the disc can be placed either inparallel or at an angle to the disc space. Unfortunately, by changingonly the angle of insertion, one cannot correct an underlying deformityof the spine. Simply changing the angle of insertion is not adequate tocompensate for a device that does not have sufficient off-center loadbearing support or structure to maintain the correction of thedeformity.

A strategy to correct lordosis in the lumbar spine has been utilized bythe Link-Charite™ and Prodisc™ lumbar disc replacement systems by usingwedge-shaped end plates. A wedge-shaped end plate has also been used inat least one case with the Bryan™ cervical disc system. However,wedge-shaped end plates are not routinely available at the present timefor cervical disc replacement systems. The strategy of usingwedge-shaped end plate(s) involves forming a differential thicknessacross the end plate. The articulation between the ball andsocket/trough or the nucleus and end plates is not altered, which is anadvantage because the complex geometry of how the prosthesis providesmotion is not altered. The disadvantage, however, is that this strategyis not forgiving if an error is made with either an overly corrected endplate or an end plate that is not corrected enough. The revision of theend plate can be difficult at the time of surgery and may even precludethe disc space from receiving a disc replacement. As most systems have acoating on the end plates that promote bony ingrowth, revision at alater date may be extremely difficult or even impossible. As there aretwo surfaces to the end plate, an outer surface that contacts the boneand an inner surface that articulates with the nucleus or core, it isconceivable that by changing the location or geometry of the innersurface, one could alter the center of rotation. This would be mostapplicable to prostheses that function as a “ball and socket”articulation. By changing the location of the “socket” or trough, thiscould alter how the prosthesis impacts alignment at the level of thedisc.

An alternate method of achieving lordotic correction is by changing thenucleus or inner core. The biggest advantage of this approach is thatthe nucleus or core can be more easily interchanged or revised.Intra-operatively, instruments can be used to gage the need for andamount of correction and the appropriate nucleus can be inserted. Bydesigning the correction into the nucleus, the surgeon is provided withflexibility and ease of insertion, and the ability for revision at alater date, which the other methods do not provide.

The invention includes a novel artificial disc that provides the normalrange of motion of the natural intervertebral disc, along with theability to correct deformity of the spine. The proposed disc allows forsemi-constrained range of motion of the functional spinal unit. It willreproduce the kinematics of the pre-operative normal spine. It willpossess maximum durability and biocompatibility, and a means forintegrating itself into the spine bony structure for long-termstability. Its insertion will be safe, simple, and ideally not addsignificantly to surgical time compared with the current procedures. Incontrast to the existing disc replacement systems, it will allow thesurgeon to correct deformity while maintaining natural kinematics of thespine.

A major advantage of this system will be that the nucleus may be easilyrevisable. For instance, in most cases where the Bryan® disc needsrevision, the entire disc, including the end plates, must be removed. Incases where the alignment of the spine changes with time, especially inchildren and young adults, this new disc replacement system will allowrevision of the nucleus, if needed.

The present invention addresses the problems associated with theartificial discs of the prior art by providing an artificial disc thatprovides for correction of spinal alignment deformity.

The artificial disc of the present invention is useful for the treatmentof degenerative disc disease including correcting spinal deformitiessuch as kyphosis, lordosis, and scoliosis.

It is an object of one aspect of the invention to provide an improvedartificial disc replacement that maintains motion at the operative leveland reduces the incidence of adjacent segment disease.

In one aspect of the invention, the artificial disc incorporates anartificial nucleus having an asymmetrical maximum vertical axis. Thepresent invention includes a non-spherical nucleus with a maximum pointof load-bearing and height in a non-central location (a differential inthe anterior/posterior height of the nucleus).

In one embodiment, the nucleus is adapted to provide lordodic correctionto a damaged spinal segment. In this case, the axis of greatest heightis positioned in the anterior part of the nucleus.

In another embodiment, the nucleus is adapted to provide kyphoticadjustment. In this case, the maximum height axis is positioned in theposterior part of the nucleus.

In yet another embodiment, the asymmetrical nucleus can be used for thetreatment of scoliosis. To achieve this, the axis of maximum height islateral (parasagittal) to the middle of the disc.

According to another aspect of the present invention, an artificialnucleus, or core, is provided for use in an artificial disc. The nucleuscomprises a body of biocompatible material, having the greatest verticalheight either at the central vertical axis or at a vertical axis otherthan the central vertical axis.

In another embodiment, the body is spherical or ovoid (egg-shaped),having convex upper and lower surfaces and a non-central maximum heightvertical axis. In an alternative embodiment, the nucleus is in the formof a truncated cylinder where the top is cut at a plane that is notparallel to the base. In another preferred embodiment, the disc isessentially circular.

It has been found that nucleus body designs with a completely roundedsurface (not necessarily spherical) have issues with reliablymaintaining correction when exposed to the variable forces of the headand neck. To address this issue, a segment or section that is flat orwhich has a contour different from the adjacent surface, can be formedin the central region of the nucleus body. This section will be referredto as a flattened section, which is meant to refer to any contour thatis not the same as the adjacent surface(s) of the nucleus. Such aflattened surface can be planar or it can have other shapes such as aslight convex or concave shape with a radius of curvature different fromthe adjacent surface. Such a flattened surface could also be in theshape of a compound curve or other complex shape. In the example ofproviding a lordotic correction, the flattened segment can be angledrelative to the superior end plate of the inferior vertebral body withthe height of the anterior part being greater than the height of theposterior part. The overall shape of the nucleus body is stillasymmetric, but the flattened segment is incorporated to provide areliable correction of the deformity. This flat segment providesstabilization of the correction by resisting misalignment moments actingthrough the nucleus. If the flattened segment is not of adequate size,there may be a tendency for the correction to disappear in the presenceof an anterior load or for a hyper-lordotic over correction in thepresence of a posterior load (during lordotic correction). An additionaladvantage of incorporating a flat segment in the nucleus is to providesurface contact over that area during small motions about the resting,neutral position of the device. This should help reduce wear on thedevice.

In another embodiment, the nucleus or core could be hemispherical inshape with a flattened inferior surface that fits in an opening ortrough formed in the lower end plate. Alternatively, the nucleus isasymmetric in that it has a greater vertical dimension or thickness onthe anterior aspect than on the posterior aspect in order to provide alordotic correction. The superior surface of the nucleus can have aflattened portion. The flattened portion may incorporate a concavesegment, but can have the other configurations as mentioned above. Theshape of the trough can be such that it defines the outer limits ofrotational or translational movement of the nucleus relative to thelower end plate. This design allows for greater ease of insertion of thenucleus without undue distraction of adjacent vertebrae because thetrough could be open at one end to allow for the nucleus to be inserted,and then a stop could be inserted in the trough to maintain the nucleusin the trough.

In another embodiment, instead of ovoid shaped nucleus, an elongated or“sausage type” shape can be used, which has spherical or ovoid endsections and a flattened or cylindrical center section. When a nucleusof this shape mates with a cylindrical bearing surface on the upper endplate, both surface and line contact are provided during lateral bendingas well as in flexion and extension. When this type of elongated nucleusis used, a corresponding end plate trough in the lower end plate can beprovided that allows for axial rotation with stops beyond the limits ofnormal motion. This trough can have the shape of a “bow tie,” “dog bone”or the like. The trough can be slightly oversized compared with thenucleus to allow limited anterior/posterior and medial/lateraltranslation. Additionally, the bearing surface of the end plate troughcan be curved upwardly at the outer limits of movement of the nucleus.This feature forces the nucleus to rise upwardly when it rotates andcause an axial distraction of the device that forces the adjacentvertebral bodies apart and loads the tissues between them, resulting ina gradual stop to the motion. The translation of the core within thetrough attempts to preserve the mobile instantaneous axis of rotation ofthe natural disc.

In another embodiment, an elongated or “sausage type” shape nucleus isshaped so that the superior surface of the nucleus possesses adepression or valley formed in the flattened section, which extendsalong the sagittal plane. This can be accomplished, for example, byremoving material from the central region of the flattened segment ofthe nucleus, creating a valley between the side portions. The sideportions are contiguous with the remaining elements of the nucleus, anddo not protrude in the vertical plane. The side portions are preferablysymmetrical about the sagittal plane.

Additionally, the trough can be open at the anterior end to allow forinsertion of the nucleus without excessive distraction of the adjacentend plates. A locking mechanism can be provided to prevent the nucleusfrom being expelled from the trough after insertion of the nucleus.

In another aspect of the invention, a novel type of end plate isprovided. Unlike other end plates, which require extensive preparationof the vertebral body surface, the present end plates have anessentially flat outer or vertebral-contacting surface that allows themto be easily inserted. In a preferred embodiment, the surface is asemi-round plate having at least one unidirectional keel for anchoringthe plate in position. The outer surface of the end plate may be treatedin a way that promotes bony ingrowth to enhance stability of the endplate in situ. In one embodiment, the outer (vertebral-contacting)surface and the inner (nucleus-contacting) surface are essentiallyparallel to each other. In another embodiment, the outer surface and theinner surface are non-parallel thereby giving the end plate anessentially wedge-like configuration. The orientation of the wide andnarrow edges of the wedge can be adjusted to provide various types anddegrees of spinal correction.

In another aspect of the invention the prosthesis comprises anartificial nucleus and at least one end plate. In this embodiment, theprosthesis comprises a superior end plate for attachment to an uppervertebral member, an inferior end plate for attachment to a lowervertebral member and a nucleus adapted to fit between the two endplates. The end plate of the invention has a generally flat surface onthe bone contacting side and the appropriate geometric receptacle on theother side for articulating with the nucleus. A central keel can beformed in the center of the inner surface of the end plate to anchor thenucleus in position. The end plate can include a stop member to preventthe prosthesis from moving toward the spinal canal. The nucleus may alsohave a maximum vertical axis that is not at the geometric center.

In another embodiment, the nucleus has an upper surface with an upperreceptacle and a lower surface with a lower receptacle. The superior endplate has a downwardly projecting protrusion or anchor that engages theupper receptacle and the inferior end plate has an upwardly extendingprotrusion or anchor that engages the lower receptacle. The prosthesismaintains an appropriate spatial relationship between adjoiningvertebrae and also permits normal range of motion of the spine. Thisembodiment can also include a receptacle that comprises a groove open atone end. The anchor on the end plate can include a central keel, whichslides into position in the groove to secure the nucleus.

Another embodiment of the invention operates like a universal joint andincorporates three anatomical axes of rotation, two of which provide forflexion/extension and lateral bending motion, while the other oneprovides for axial rotation. These axes of rotation are accomplished bythe use of a pair of two cylinders that can rotate relative to eachabout a central post.

In another embodiment, one of the plates has a central post that engagesthe other plate, and an annular core positioned around the central postthat is formed of a resilient material. The core can be asymmetrical andengage both plates to provide necessary deformity correction. The corecan engage the end plates to provide the desired angle between theplates for deformity correction, with the central post engaging theother plate when the load exceeds a predetermined limit. Or, the postcan engage the other plate with the core engaging the other plate tomaintain the plates at the desired angle relative to each other whenapplied forces tend to change the relative angle of the plates.Alternatively, the core could be replaced by two or more discretespacers for performing the same function.

In another aspect of the invention, the nucleus can utilize materialdeformation to accomplish the desired ranges of motion. The shape of thematerial can be used to provide a restoring force for deformitycorrection. In order to achieve these results, material can be removedfrom various parts of the core to change the modulus of elasticity ofthe core at selected locations, or material having variable elasticmoduli could be used. In this way, different forces and motions can beprovided though the design of the core.

The end plates can be provided with features that act as stops outsideof the desired range of motion, which allow for anatomically-derivedgradual stopping. This result can be achieved by forming one or morecamming surfaces in or on one of the end plates and providing aco-operating member on the other end plate for engaging the cammingsurface. The camming surface has a gradual curve on its inner surface.During relative movement between the end plates, the camming surface isengaged by the cooperating member, which results in an axial distractionof the end plates and provides a soft tissue assist to prevent a hardstop. Alternatively for rotational movement, cooperating cammingsurfaces can be provided so that distraction will occur when one endplate rotates relative to the other one.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set fourth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1A illustrates an spherical artificial disc nucleus with themaximum central axis in the geometric midline of the nucleus;

FIG. 1B illustrates the nucleus of FIG. 1A, with an offset maximumvertical axis that provides 3° of correction;

FIG. 1C illustrates the nucleus of FIG. 1A, with an offset maximumvertical axis that provides 6° of correction;

FIG. 2A illustrates an asymmetrical artificial disc nucleus with themaximum central axis in the geometric midline of the nucleus;

FIG. 2B illustrates the nucleus of FIG. 2A with an offset maximumvertical axis that provides 3° of correction;

FIG. 2C illustrates the nucleus of FIG. 2A with an offset maximumvertical axis that provides 6° of correction;

FIG. 3 is a top view of the embodiment of the artificial disc nucleusshown in FIG. 1A;

FIG. 4 is a perspective view of the embodiment of the artificial nucleusshown in FIG. 1A;

FIG. 5 is a perspective view of the embodiment of the artificial nucleusshown in FIG. 2A;

FIG. 6 is a perspective view of an outer surface of an end plate;

FIG. 7 is a perspective view of an inner surface of an end plate;

FIG. 8 is a front view of an end plate;

FIG. 9 is a front view of a spinal disc device with the nucleus shown inFIG. 1A;

FIG. 10 is a side view of the spinal disc device of FIG. 8;

FIG. 11 is a front view of a spinal disc device with the nucleus shownin FIG. 2A;

FIG. 12 is a side view of the spinal disc device of FIG. 8;

FIGS. 13A and 13B illustrate an embodiment of an artificial spinal discprosthesis where the end plates may be adapted for lordotic correction;

FIGS. 14A, 14B, and 14C illustrate other embodiments where the endplates can be adapted for lordotic correction;

FIG. 15 is a side view of another embodiment which provides for alldirections of movement;

FIGS. 16A and 16B illustrate the two sections of the nucleus of theembodiment of FIG. 15;

FIGS. 17 and 18 illustrate another embodiment of the invention in whichthe nucleus is formed of upper and lower sections with an intermediatesection;

FIG. 19 illustrates another embodiment of the invention in which thenucleus is cut in half and has a flat lower inferior surface;

FIG. 20 is a schematic view of the nucleus of FIG. 19;

FIG. 21 illustrates a modification of the embodiment of FIG. 19;

FIG. 22 is a an underside view of the nucleus of FIG. 21;

FIG. 23 is a schematic view of the nucleus of FIG. 21;

FIG. 24 illustrates a modification of embodiment of FIG. 19;

FIGS. 25-31 illustrate another embodiment of the invention in which thenucleus is elongated with a flattened section in the center;

FIGS. 32 and 33 illustrate another embodiment of the invention whichutilizes a universal joint;

FIGS. 34-36 illustrate another embodiment of the invention in which aresilient ring and a post provide for relative motion between the endplates;

FIG. 37 illustrates a modification of the embodiment of FIG. 34;

FIGS. 38 and 39 illustrate another embodiment of the invention in whichthe nucleus is shaped to provide medial/lateral correction; and

FIGS. 40-43 illustrate another embodiment of the invention in which theend plates are provided with stops outside the normal range of motion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to systems and methods for partially orwholly replacing diseased or injured joints with artificial jointprostheses. Those of skill in the art will recognize that the followingdescription is merely illustrative of the principles of the invention,which may be applied in various ways to provide many differentalternative embodiments. This description is made for the purpose ofillustrating the general principles of this invention and is not meantto limit the inventive concepts in the appended claims.

In its proper, healthy alignment, the spine follows natural curves,which promote proper sagittal and coronal balance (flexibility) andallow for balanced load sharing between the vertebrae. These curvesinclude the cervical, thoracic, lumbar and sacral regions of the spine.Naturally, in order to accommodate a curve, there must be some variationin the angle of articulation between the functional spinal units and theheight of an intradiscal space. The cervical and lumbar regions arenaturally lordotic, or curved convexly in the anterior direction. Atdifferent segments along the spine, there are typically differentheights for the vertebral bodies and the intradiscal space. In addition,the intradiscal space and vertebral body height may be different fordifferent people.

Each intradiscal space has anterior and posterior regions. An artificialdisc in the cervical, thoracic and lumbar regions that maintain the sameheight from the anterior to the posterior may promote an abnormalalignment, resulting in additional stress at the anterior or posteriorportions of an adjacent disc. It may also result in an uneven loaddistribution across the device and cause an excessive amount of relativemotion, wear debris and early failure.

As used herein, the terms, nucleus and core are used interchangeably torefer to an artificial intervertebral device that replaces a damagednatural spinal disc. The artificial core may be provided alone or incombination with a superior end plate for attachment to an uppervertebra or an inferior end plate for attachment to a lower vertebra orboth.

The terms “upper” and “lower” are used herein to refer to the vertebraeon either side of the disc to be replaced, or a surface on a part in theposition shown in the referenced drawing. A “superior” plate is affixedto an upper vertebra and an “inferior” plate is affixed to a lowervertebra of a functional spinal unit.

The terms vertical and horizontal are used herein relative to a standinghuman being in the anatomical position. The term “anterior” refers tothe region towards the front and the term “posterior” refers to theregion towards the back. The term “sagittal” refers to regions on eitherside of the central midline axis of a standing human being.

The term “asymmetrical” is used herein to refer to an axis of maximumheight that is not placed centrally or to a nucleus or total discreplacement (TDR) not having its maximum vertical axis placed centrally.In other words, the maximum height is not situated or pivoted at acenter line of symmetry so that the TDR comprises regions that are notexactly the same in shape or size as other regions on the other side ofa line of symmetry. The location of maximal load bearing is located in anon-central location.

In one embodiment of the present invention, an artificial disc comprisesa nucleus that is not geometrically symmetrical. The disc may have amaximum vertical axis that is not located at the geometric center of thedisc. The maximum vertical axis may be located toward the front of thedisc, the rear of the disc or on one side of the disc. The positioningof the maximum vertical height and load bearing capability is chosendepending on the type of deformity that needs to be corrected. Thepresent invention also provides methods for the treatment ofdisc/vertebral body disease, lordosis, kyphosis and scoliosis using anasymmetric artificial disc.

One advantage of the present invention is that the “nucleus” or core maybe interchanged and revised intra-operatively and post-operatively.Instruments can be used to gauge the need for and amount of correctionand the appropriate implant can then be inserted. By introducingcorrection into the nucleus, the surgeon benefits from flexibility, easeof insertion and revisability that present systems do not provide.

Artificial discs of the present invention can be provided with variousdegrees of deformity correction. For this aspect of the invention, thesurgeon can choose a disc having the appropriate correction for thepatient. Thus, a method of treating a spinal deformity is provided. Thismethod comprises preparing a spinal segment for implantation of anartificial disc, determining the desired angle of the intervertebralspace, selecting an artificial nucleus having the desired dimensions,affixing a superior end plate to the upper vertebra, affixing aninferior end plate to the lower vertebra and inserting the selectednucleus between the superior and inferior end plates. Alternatively, andthe assembled unit of end plate-nucleus-end plate may be inserted inunison. The configuration of the nucleus in this pre-assembled constructcan be determined by the intra-operative measurement tools, or withpre-operative calculations. Pre-operative planning techniques andinstruments may also be able to determine the size and orientation ofthis device for insertion.

A major advantage of the present system is that the artificial disc canbe more easily and rapidly inserted and the nucleus can be changed orrevised in accordance with the magnitude of the deformity beingcorrected. This is especially useful in children and young adults wherethe alignment of the spine changes over time.

In one embodiment, an asymmetric nucleus adapted for lordotic correctionof the cervical spine is provided. The surgeon can restore lordosis tothe cervical spine while maintaining motion. The nucleus may be composedof a low friction elastomer such as polyurethane,polycarbonate-polyurethane, a polymer such as polyethylene (particularlyultra-high molecular weight polyethylene), a suitable ceramic, metals ormetal alloys such as titanium or a titanium alloy,chrome-cobalt-molybdenum (CoCrMo), cobalt 28 chromium molybdenum, cobaltchrome, stainless steel, or other suitable materials. It has a generallycircular geometric design, with varying degrees of lordosis incorporatedinto it by utilizing an axis of maximum height anterior to the geometriccenter of the nucleus. The anterior height of the nucleus varies,depending on the extent of lordotic correction needed. The nucleus isavailable in various lordotic angles, e.g. 0, 3° and 6°, as well asdiffering heights (e.g., 4, 6 and 8 mm). Before deciding on the finalnucleus size, a set of instruments or other means can be used to gaugethe need for lordotic correction.

The nucleus slides between a superior end plate and an inferior endplate. The nucleus can be maintained in position using various types ofconnectors. For example, in one embodiment, the convex surface of thenucleus has a midline groove to allow the nucleus to slide into placebetween the positioned end plates. A central keel on the concave surfaceof the end plate is received in the groove of the nucleus. It isapparent that other types of connections can be used to maintain thenucleus in position. For example, a tooth and lock system or a pop-insystem could be used.

A number of embodiments of the nucleus and artificial disc of thepresent invention are illustrated in the appended drawings. In oneaspect of the invention, correction of spinal segment alignment isprovided by an artificial nucleus which has the shape of a truncatedcylinder or which is generally spherical or ovoid in shape, wherein thetwo halves on the arc on either side of a central axis are notsymmetrical. In other words, the curvature is not geometrically parallelor symmetric.

In one embodiment, the implant consists of three pieces. The end plateswill be made in differing sizes to accommodate differences in anatomy.These may be fabricated of titanium or a titanium alloy,chrome-cobalt-molybdenum (CoCrMo), cobalt 28 chromium molybdenum, cobaltchrome, stainless steel or other materials suitable for spinalprosthetic inserts.

The end plates can have two distinct surfaces. The flat surface of eachend plate, which contacts the vertebral body end plate, is capable ofaccommodating bony ingrowth and incorporates a suitable coating, such asporous titanium, a calcium phosphate, or includes other types of knownsurfaces that promote bony ingrowth for long-term stability. The endplates can also have one or more parasagittal keels that provideimmediate fixation. In one embodiment of the invention, a pair ofparallel keels can be formed on the outer surface of one of the endplates, and a single, centrally-located keel can be formed on the outersurface of the other end plate. The other (inner) surface of the endplates can have a contour that corresponds with the geometric shape ofthe nucleus to form a bearing surface that allows for optimalarticulation and wear characteristics with respect to the nucleus. Inthe middle of this bearing surface, there can be a single, central keel,which provides a constraint for the nucleus against excessivetranslation and range of motion. The nucleus can have a circulargeometric design, with a midline groove to allow the nucleus to slideinto place between the positioned end plates. A central keel on theconcave surface of the end plate would fit into the groove of thenucleus. Before deciding on the final nucleus size, a set of instrumentscould be inserted to confirm the lordotic correction, but these may alsobe used as confirmation for other types of pre-surgical planningtechniques and instrumentation. Alternatively, intra-operativeinstruments may be used as confirmation for other types of pre-surgicalplanning techniques and instrumentation.

FIGS. 1A to 1C illustrate various examples of artificial disc nucleiwhere the nucleus is symmetrical, with a maximum central axis in thegeometric center 20 of a nucleus 10. The reference letters A and Pillustrate the anterior and posterior orientation, respectively, of thenuclei 10, 14 and 18. The nucleus 10 is generally spherical in shape andis truncated with a flattened portion 22A on the upper side of thenucleus 10 and another flattened surface 22B on the lower side. Thenucleus also has upper and lower curved surfaces 24A and 24B,respectively, and a circumferential wall 26.

The flattened surfaces, as described above, can be advantageous becausewhen the nucleus has a completely rounded surface, it cannot reliablymaintain correction when exposed to the variable forces of the head andneck. A flattened surface incorporated into the central region of thenucleus can be used to solve this problem. The flattened surfaces have acontour different from the adjacent surface, and are formed in thenucleus body. The terms “flattened section” or “flattened surface” areused interchangeably and are meant to refer to any contour that is notthe same as the adjacent surface(s) of the nucleus. Such a flattenedsurface can be planar or it be slightly convex or concave and have aradius of curvature different from the adjacent surface. Such aflattened surface could also be in the shape of a compound curve orother complex shape.

This flattened surface can be angled relative to the superior end plateof the inferior vertebral body (or vice versa, or both), with the heightof the anterior end being greater than the height of the posterior endwhen lordotic correction is sought. The overall shape of the core canstill be asymmetric, but the flattened surface can be incorporated toprovide a reliable correction of the deformity. This flattened segmentprovides stabilization to resist the moments acting through the nucleus,i.e., if the flat is not of adequate size, there may be a tendency forthe correction to disappear in the presence of an anterior load or for ahyper-lordotic over correction in the presence of a posterior load(during lordotic correction). Another advantage of the flattened segmentis to provide surface contact over that area during small movementsabout the, neutral position of the device, which could help reduce wearon the device.

FIG. 1A illustrates a nucleus 10 that has not been adapted for lordoticcorrection because the upper and lower surfaces 22A and 22B are parallelto each other. In this nucleus, the axis 20 of greatest height falls inthe center of the disc. In FIG. 1B, a nucleus 14 that provides 3° ofcorrection is illustrated. This nucleus provides for lordoticcorrection. FIG. 1C illustrates another artificial disc nucleus 18having a greater degree of deformity correction. When deformitycorrection is provided as shown in FIGS. 1B and 1C, the geometric centerof the nucleus may shift to a location that is offset from the axis 20.

If the anterior/posterior directions are reversed, it provides akyphotic correction. If the nucleus is rotated 90 degrees, a scolioticcorrection is provided. In the illustration in FIG. 1C, the maximumvertical axis 20 is positioned to provide a correction of 6°. It isapparent that the nucleus can be adjusted to provide various degrees ofcorrection and, in certain cases, if no degree of correction is needed.Alternatively, only one of the halves of the nucleus 10 may have aflattened portion, with the other half having an outer surface that iscurved.

In FIGS. 2A through 2C, asymmetrical ovoid embodiments of an artificialnucleus are shown. The nucleus comprises upper and lower surfaces 22Aand 22B, which are “flattened” by virtue of the ovoid shape of thenucleus, upper and lower curved surfaces 24A and 24B, and acircumferential center portion 26. In the embodiments shown in FIGS. 2Band 2C, the maximum height axis 16 is asymmetrical with the geometriccenter 12 of the disc. In the nucleus shown in FIG. 2A, where there isno correction, the maximum vertical height is at the central verticalaxis 12. In the nucleus shown in FIG. 2B, the maximum vertical axis 16is positioned to provide an angle of correction of 3°. In the nucleusshown in FIG. 2C, the maximum vertical axis 16 is positioned to providean angle of correction of 6°.

FIG. 3 is a top view of one example of a nucleus. This nucleus 40comprises a central convex or flattened region 42, which includes agroove or slot 44. This groove or slot 44 enables the nucleus to slideonto the central keel or anchor of an end plate (not shown). While thenucleus 40 is shown as essentially circular, it is clearly apparent thatit may take on other shapes such as an ovoid or ellipsoid shape. It isalso clearly apparent that other types of anchor receiving means can beused. For example, the shape of the groove may vary or a snap-in orbayonet or dog-bone type of receptacle can be provided to anchor thenucleus in position. Those practiced in the art can provide additionallocking methods including the addition of one or more parts to the corethat provide an anchor.

For deformity correction, the nucleus may take the form of a truncatedcurved body as shown in FIG. 4. For this embodiment, the nucleus 50 hasan upper surface 52 that terminates in essentially flattened planar top54. A slot 56 or a groove or opening of another appropriate shape, canbe formed in upper surface 52 for receiving an anchor formed in the endplate. The lower surface 58 is typically an inverse of the uppersurface. However, instead of being truncated with a flat surface asshown in FIG. 4, the bottom surface could be asymmetrically spherical orovoid in shape.

Alternatively, the nucleus may be circular, ovoid or egg-shaped having anon-central maximum vertical axis as shown in FIG. 5. In anotherembodiment, the nucleus could be essentially circular or asymmetricallyspherical.

FIG. 5 illustrates an artificial nucleus 60 where the upper surface 62is an asymmetric convex surface. Again, either the top or the bottom orboth surfaces may be asymmetric.

For illustrative purposes, the nuclei in the figures have been shownadapted for lordotic correction. It is clearly apparent that the nucleuscan have an asymmetric maximum height at the front (anterior), the rear(posterior) or the side (lateral). The asymmetrical nucleus of thepresent invention can be used to correct for various types of spinalmisalignment including sagittal and coronal deformity.

The novel corrective nucleus of the present invention may be providedalone or it may be provided in combination with an upper end plate, alower end plate or both an upper and a lower end plate.

FIGS. 6 through 8 illustrate an exemplary artificial end plate 70 thatcan be used in conjunction with the nucleus to provide a novelartificial disc unit. An artificial end plate according to the presentinvention comprises an inner surface with a concave bearing surface forreceiving the convex surface of an artificial disc. The outer, or bonecontacting, surface is essentially flat.

To accommodate some previously known end plates, it was necessary tospend a significant amount of surgical time to prepare the vertebrae tothe appropriate shape to accommodate the artificial end plate. FIG. 6shows an end plate 70 with a flat outer surface 72 that enables the endplate to slide on the surface of the vertebra. One or moreunidirectional keels 76 are formed on the outer surface 72 to providefor immediate fixation. The keels may be placed centrally orparasagittally. Fixation can be enhanced by incorporating onto the outersurface 72 a suitable coating 74, such as porous titanium, a calciumphosphate or the like, to promote bony ingrowth for long term stability.

A stop member 78 can be provided at the anterior edge 80 of the endplate. The stop member prevents the prosthesis from migratingposteriorly and possibly impinging on the spinal cord. An essentiallysemi-circular wall 82 joins the outer surface of the end plate to theinner surface. The thickness of 82 may vary with increased thicknessanteriorly, posteriorly or parasagittally, as discussed further below.The inner surface 84 is shown in greater detail in FIG. 7.

The inner surface 84 of the end plate articulates with the nucleus. Inthe embodiment shown in FIG. 7, this inner surface has a concave region86, which receives the nucleus. An anchor 88 is provided in the centerof the concave region 86 for positioning the nucleus and preventing itfrom migrating. The anchor 88 can be generally rectangular in shape withrounded edges, as shown, avoiding premature wear and cutting into thenucleus. FIG. 8 illustrates a front view of the end plate showing theouter surface 72 having two parasagittal keels 76 and the inner surface84 having a concave region 86 and a central anchor 88.

FIGS. 9-12 illustrate a nucleus and end plates described above assembledinto a TDR implant. FIGS. 9 and 10 show the use of a nucleus 96 with atruncated cylinder shape and a flattened portion 97 on the superior sideof the nucleus as described above, in conjunction with FIGS. 1A-1C, andFIGS. 11-12 show the same design with a nucleus 96 having an ovoid shapeas shown in FIGS. 2A-2C. In these figures, a complete spinal discprosthesis 90 comprising a superior end plate 92, an inferior end plate94 and an artificial disc nucleus 96 is provided. The end plates andnucleus can be provided in different sizes to accommodate differences inanatomy. The end plates and various nuclei can be provided in a kit tothe surgeon so that the appropriate sized components can be selected andused when the final size is determined. The end plates may be fabricatedof titanium or titanium alloy, chrome-cobalt-molybdenum (CoCrMo), cobalt28 chromium molybdenum, cobalt chrome, ceramics or other materialsuitable for spinal prosthetic implants.

The end plates have two distinct surfaces. The outer surface 98 is thesurface that contacts the vertebral end plate. The outer surface isessentially flat enabling it to easily contact the surface of thenatural vertebral end plate. The flat surface can be porous andincorporate a suitable treatment, such as porous titanium, a calciumphosphate or other types of known treatments such as coatings, plasmasprays, and structural changes to the surface, that promote bonyingrowth or ongrowth for long-term stability. At least one parasagittalkeel 100 is formed on the outer surface of each end plate to provideimmediate fixation.

As shown in FIGS. 9-12, three parasagittal keels 100 are aligned witheach other and located along both sides of the outer surface of the endplates. Alternatively, as shown FIG. 9A a similar end plate design withan upper end plate 92 and a lower end plate 94 have an offset keelconfiguration with a pair of aligned parasagittal keels 100A formed onthe outer surface of the upper end plate and a centrally-located row ofaligned keels 100B formed on the outer surface of the lower end plate94. This latter arrangement is believed to be advantageous because, withthe upper and lower keels being offset from each other, the end platesshould have greater stability and result in less stress on a vertebrawhere multiple implants are used.

Referring back to FIGS. 9-12, the inner surface 102 of each of the endplates has a concave region 103 or bearing surface that articulates withthe nucleus. An anchoring protrusion 104 projects outwardly from theconcave region, which provides an anchor for the nucleus and restrictsposterior translation. Both the superior and the inferior end plateshave flanges 106 for preventing the end plates from migrating into thespinal canal. The end plates can have holes 107 for allowing the endplates to be connected to the adjacent vertebrae through either metallicor bioabsorbable screws (not shown) that can be inserted through holes107. FIGS. 9 and 11 illustrate front views of the prosthesis and FIGS.10 and 12 illustrate side views.

In another aspect of the invention, shown in FIGS. 13A-13B and 14A-14C,spinal deformity can be addressed by providing an artificial spinal discprosthesis where correction is provided in the end plates. Correctiveend plates may be provided alone, in combination with a symmetricalartificial nucleus that has flattened surfaces as described above onboth the top and bottom of the nucleus, as shown in FIGS. 13A-13B, or incombination with an asymmetrical nucleus that has flattened surfaces asdescribed above on both the top and bottom of the nucleus, as shown inFIGS. 14A-14C.

Correctional end plates are shown in FIGS. 13A-13B and 14A-14C. Thedegree of correction can be achieved by altering the inner(nucleus-contacting) side of the end plate or the outer(vertebral-contacting) side of the end plate. As shown in FIGS. 13A-13B,the end plate 110 comprises an outer (bone-contacting) surface 112, aninner surface 114, and a perimeter wall 116 connecting the outer andinner surfaces. The height of the perimeter wall 116 may vary accordingto the degree and type of correction required. For example, FIG. 13Billustrates an end plate adapted for a greater degree of correction thanthe end plate of FIG. 13A. The positioning of the variable height can beadjusted to treat different conditions such as lordosis, kyphosis orscoliosis. The inner surface may be shaped to receive the nucleus, andthe height of the end plate can be adjusted according to the degree ofcorrection required.

Alternatively, as shown in FIGS. 14A-14C, the outer surface 120 and theinner surface 122 may be essentially planar and the height is adjustedas the outer and inner surfaces become increasingly non-parallel as aresult of variation in the height of the perimeter wall 124. FIGS. 14Athrough 14C illustrate increasing degrees of correction, respectively.An advantage of having an essentially planar outer, orvertebral-contacting, surface is that the device is easier to insert andrequires less operating time to prepare the vertebral surface ascompared to traditional artificial disc devices.

FIGS. 15, 16A and 16B illustrate another embodiment of the invention,which provides for all directions of movement, flexion/extension,lateral bending, and rotation about the symmetrical axis. In thisdesign, the nucleus 130 is formed in two sections 130A and 130B. A post132 is formed on the inner surface of one section 130A, and fits in anopening 134 that is formed on the inner surface of the other section130B to provide for relative rotational movement between the twosections 130A and 130B. The post 132 and opening 134 can be formed oneither section of the nucleus 130. The post and opening can be of anysuitable size, and can be perpendicular to the opposing surfaces of thenucleus sections 130A and 130B, or be tilted at an angle off horizontalto orient the axis of axial rotation with the anatomically correct axisand provide a deformity correction.

In this configuration, the contact surfaces between the nucleus 130 andend plates 136 and 138, are designed to have the same correspondingasymmetrical contours at the preferred angle between them, as shown inFIG. 15. Because there is only relative movement between the nucleus andthe end plates in the anterior/posterior and medial/lateral directions,greater surface contact between the nucleus and the respective endplates is possible in order to transmit rotations of the end plates tothe nucleus so that the two halves 130A and 130B, of the nucleus 130will rotate with respect to each other, rather than having the endplates 136 and 138, rotate on the outer surface of the nucleus 130.

FIGS. 17 and 18 show another embodiment of the invention where insteadof forming the nucleus 130 of a single piece of material, it can beformed of upper and lower sections 130A and 130B, with an intermediatesection 140, that is either flat or wedge-shaped as shown in FIG. 17,fixed to the upper and upper and lower sections. The intermediatesection 140 can provide the nucleus with the appropriate degree ofcorrection as shown in FIG. 18, instead of providing wedge-shaped endplates as discussed above. In a related embodiment of the invention, thenucleus 130 is essentially cut in half and has a flat inferior surface.This can be applied to the embodiment seen in FIGS. 17 and 18, where thesection 130B is removed, leaving the inferior surface of intermediatesection 140 articulating with the inferior end plate. By varying theconfiguration of the intermediate section 140, deformity correction canbe achieved.

FIGS. 19 and 20 show another embodiment of the invention where thenucleus 130 is essentially cut in half and has a flat lower inferiorsurface. This shape can be used to resist expulsion of a nucleus with anovoid/asymmetric shape, which could occur when the ovoid shape of thenucleus causes the end plates to tilt relative to each other to providecorrection. As shown, the bottom surface of the nucleus 130 is flat andis formed with a circular opening 134 that is shaped and positioned toreceive a post 136 formed on the opposing surface of the lower end plate138 for allowing relative rotational movement between the nucleus 130and the end plate 138. Alternatively, the nucleus could have the flatsurface and opening 134 on its upper or superior surface, instead ofbeing on the lower surface as shown. In this embodiment, the nucleus ispreferably asymmetrical as shown in FIG. 19.

A modification of the configuration in FIGS. 19 and 20, is shown inFIGS. 21 through 24, where the nucleus 130 is positioned in a slot ortrough 142 formed in the upper surface of the lower end plate 138. Asshown in FIG. 21, the undersurface of the upper end plate 136 iscontoured to match the nucleus. Alternatively, as shown in FIG. 24, theundersurface of the end plate 136 can be flat and engage a flattenedupper surface 146 of the nucleus 130.

The trough can be larger than the nucleus in both the anterior/posteriorand medial/lateral directions to allow for a desired amount oftranslation in those directions as shown by the arrows A and B in FIG.22. The trough can be open on the posterior or anterior end to allow thenucleus to be inserted simply by sliding it into the trough, as shown bythe arrow A in FIG. 21. In this way, the nucleus can be inserted withoutundue distraction of adjacent vertebrae. The nucleus can be preventedfrom moving out of the trough by providing a stop 144 of any suitablesize and shape. FIG. 23 is a schematic view of the nucleus 130 that isinserted in the trough in FIG. 21.

Another embodiment of the invention is shown in FIGS. 25-31, where thenucleus 130 is elongated, with a flattened section 150 that is a partialcylinder with curved sections 152 and 154 on both sides of the flattenedsection. It is believed that this design, when mated with a cylindricalsurface 156 on the interior of the upper end plate 136, shown in FIG.29, will provide better wear characteristics because it will havesurface contact during medial/lateral bending and line contact duringflexion/extension.

The elongated shape of the nucleus 130 is illustrated in FIGS. 25 and26, which show that the nucleus has a round cross section with constantmedial-lateral radius from anterior to posterior (A-P), with the flatsection 150 in the middle being oriented to provide a correction angleas described above, for the flatted portions on the other embodiments ofthe nucleus. The interior surface 156 of the upper end plate 136 has acylindrical shape with the same constant radius in theanterior/posterior direction as the nucleus.

In the neutral position, the cylindrical surface 156 mates with theflattened section 150 of the nucleus 130, and sits at an angle thatprovides a deformity correction as shown in FIG. 31. In this position,there is surface contact between the end plate 136 and the nucleus 130.During medial/lateral bending, there is also surface contact between theend plate and nucleus. During flexion/extension, with or without lateralbending, there is line contact between the end plate and nucleus. Thisconfiguration of core and end plate will always have line or surfacecontact, thus reducing the wear potential from point contact in some ofthe previous designs.

The elongated shape of the nucleus 130 allows for the end plate 138 tohave a trough 157 in the shape of a “bow tie” as shown in FIGS. 27 and28. This shape allows for axial rotation with stops beyond the limits ofnormal motion. The shape is oversized relative to the nucleus 130 by anappropriate amount to allow limited anterior/posterior andmedial/lateral translation. Additionally, the bottom surface of thetrough 157 can be rounded upwardly at the medial/lateral sides in FIG.30 (not shown), so that as the nucleus 130 rotates it is “cammed” upcausing a distraction of the device that forces the vertebral bodiesapart and loads the tissues between them resulting in a gradual stop tothe motion. Translation of the nucleus 130 within the trough 157 willtend to preserve the mobile instantaneous axis of rotation of thenatural disc.

FIGS. 32 and 33 show another embodiment of the invention, which utilizesa universal joint formed of a pair of cylinders 160 and 162 that rotaterelative to each about a central post 164 that projects from one of thecylinders 162 and engages an opening 166 in the other cylinder 160. Thecylinders 160 and 162 are oriented perpendicular to each other andengage cylindrical surfaces 168 and 170, respectively, in the adjacentend plates 136 and 138. This design provides for three anatomical axesof rotation. Because of the independence of each axis of rotation, anycorrection provided by the shape of the nucleus that is formed of thetwo cylinders will result in rotation to compensate for the correctionand a return to the uncorrected neutral position. Alternatively, thecylinders 160 and 162 may be shaped similarly to the elongated nucleus130 shown in FIGS. 25-27, or another suitable shape with a flat inferiorsurface.

Another embodiment of the invention is shown in FIGS. 34-36, where aresilient ring 172 and a post 174 that has a rounded top portion 176provide for relative motion between the end plates and for the desiredangle of correction. The ring 172 is shown in detail in FIG. 35. Thering 172 can be wedge shaped as shown in order to provide the desiredamount of correction, or it can be flat (not shown) if no correction isdesired. A projection 180 can be formed on the upper surface of thelower end plate 138 to mate with an opening 182 in the ring 172 in orderto prevent the ring 172 from moving relative to the lower end plate oncethe ring is in its desired position.

The upper end plate 136 has a cavity 178 that can be contoured to matchthe shape of the rounded top portion 176. The ring 172 is shaped so thatthe end plate 136 will ride on the ring 172 during “normal” ranges ofmotion, or through regular activities. However, when the normal rangesof motion are exceeded, then the ring 172 will compress and the upperend plate 136 will engage the post 174 causing the adjacent vertebrae todistract and thereby provide a gradual stopping motion or“anatomically-derived gradual stopping.” Alternatively, the post 174could be designed to serve as the primary load carrying part of thearticulation by riding in the cavity 178. In this design, the deformitycorrection force is only provided by compressing the ring 172. Thisdesign would have the advantage of reducing material stresses in theelastomer ring and creep.

As shown in FIG. 37, instead providing the ring 176, the same resultcould be achieved by providing two or more stops 184A and 184B, formedof a resilient material, between the two end plates. The stops 184A and184B can be mounted on the upper surface of the lower end plate 138. Oneof the stops 184A can project upwardly a greater distance than the otherstop 184B in order to provide the desired correction.

Another embodiment of the invention is shown in FIGS. 38 and 39, where anucleus 186 is provided that is formed of a resilient material that isshaped so that the nucleus provides medial/lateral rotation, butrequires deformation of the material during flexion and extension. Thisis accomplished by proving a central portion 188 that is spherical orovoid in shape and “flattened” adjacent end portions 190A and 190B thatare cylindrical, which extend the flattened end portions around thecircumference of the nucleus at both ends. The upper end plate has acavity (not shown) that has a contour that is similar in shape to thenucleus 186. A trough (not shown) similar to the one in FIGS. 27 and 28can be formed in the lower end plate 138.

For medial/lateral movement in the direction of the arrows A-A, theupper and lower end plates will rotate relative to each other throughrotational movement of the upper end plate on the nucleus 186. However,flexion/extension in the direction of arrows B-B will occur only throughdeformation of the nucleus 186. Alternatively, the nucleus 186 can berotated 90° on the lower end plate 138 so that so that the end plate 136will rotate on the nucleus during flexion/extension and the nucleus willdeform during medial/lateral movement. The end portion 190A has a largerdiameter than the end portion 190B to provide for the desired amount ofcorrection. As shown, the nucleus has been shaped so the resilience ofthe nucleus varies over its length. However, the nucleus could be formedof materials having varying degrees of resiliency along its length toachieve the same results.

FIGS. 40-43 illustrate another embodiment of the invention where the endplates 136 and 138 are provided with stops outside of the normal rangeof motion, which also utilize the concept of “anatomically-derivedgradual stopping” discussed above in conjunction with FIGS. 34 and 35.This type of stop can be added to any design that employs the use of endplates. This aspect of the invention is based on duplicating how thehuman body moves and then designing the cooperating surfaces to mimicthose motions as closely as possible. As shown in FIG. 40, the end plate136 has a post 200 on its lower surface that engages pocket 202 formedin the upper surface of the lower end plate and 138. Preferably, a pairof posts and pockets are provided on opposite sides of the nucleus 130.

As shown in FIGS. 40-43, the pocket 202 has a slot 204 in it with acurved surface 206 that is engaged by the lower end 208 of the post 200.As the end plates 136 and 138 move in the anterior/posterior directionrelative to each other during extension/flexion, the lower end 208 ofthe post rides along the curved surface 206. As the post reaches theouter limits of travel the lower end 208 will begin riding up thegradually curved section of the surface 208, which causes distractionbetween adjacent vertebrae as illustrated by the arrow A in FIG. 43 andloads the tissues between them, resulting in a gradual stop to themotion.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

The invention claimed is:
 1. A nucleus for an artificial disc forreplacing the natural disc of a human spine and being adapted to fitbetween and move relative to first and second end plates that areattached to adjacent vertebrae, the nucleus comprising: a nucleus bodyhaving superior and inferior bearing surfaces for engaging cooperatingbearing surfaces formed on the first and second end plates,respectively, the superior bearing surface facing opposite the inferiorbearing surface; said superior bearing surface having a first curvedportion convexly curved in the sagittal plane, wherein a first planarportion is formed on the first convex curved portion for engaging androtationally articulating relative to a cooperating bearing surface onthe first end plate to maintain a correction orientation of at least oneof the end plates relative to the nucleus, the first planar portionconfigured to extend across the intersection of the coronal and thesagittal planes, the coronal and the sagittal planes defined relative tothe nucleus body such that the intersection defines a verticallyextending geometric center axis of the nucleus body, the nucleus furthercomprising a circumferential flange positioned between the inferior andsuperior bearing surfaces, the flange projecting radially away from thenucleus body.
 2. The nucleus of claim 1, wherein the second bearingsurface is planar.
 3. The nucleus of claim 1, wherein the nucleus bodyis asymmetrical in the sagittal plane along the midline of the nucleusbody, relative to the vertically extending geometric center axis of thenucleus body.
 4. The nucleus of claim 1, wherein the nucleus has ahorizontal axis and the first planar portion is formed at an angle otherthan parallel relative to the horizontal axis for maintaining thecorrection orientation of the artificial disc when the nucleus engagesthe end plates.
 5. The nucleus of claim 4, wherein the angle is between3 and 6 degrees.
 6. The nucleus of claim 1, wherein the nucleus body hasa second curved portion formed on the second bearing surface.
 7. Thenucleus of claim 6, wherein a second planar portion is formed on thesecond curved portion.
 8. The nucleus of claim 1, wherein the nucleusbody is formed of a polymeric material.
 9. The nucleus of claim 1,wherein the nucleus body is formed of a resilient material.
 10. Thenucleus of claim 1, wherein the nucleus body further comprises acircumferential wall positioned between and separating the superiorbearing surface and the inferior bearing surface.
 11. The nucleus ofclaim 1 and further including first and second end plates adapted to beattached to adjacent vertebrae, the nucleus body being positionedbetween the end plates to form a total disc replacement.
 12. A nucleusfor an artificial disc for replacing the natural disc of a human spineand being adapted to fit between and move relative to first and secondend plates that are attached to adjacent vertebrae, the nucleuscomprising: a non-resilient nucleus body having a first bearing surfaceand a second bearing surface, the first and second bearing surfacesconfigured to engage and articulate relative to the respective first andsecond end plates, the first bearing surface comprising a first curvedportion convexly curved in the sagittal plane, a planar first portionformed on the first convex curved portion, the planar first portionconfigured to extend across the intersection of the coronal and thesagittal planes, the coronal and the sagittal planes defined relative tothe nucleus body such that the intersection defines a verticallyextending geometric center axis of the nucleus body; a planar secondportion formed on the second bearing surface, the planar first portionopposite the planar second portion, a maximum height axis extendingbetween the first and second planar portions at the maximum height ofthe nucleus body; wherein the nucleus body is asymmetrical in thesagittal plane, the maximum height axis offset from the geometric centeraxis of the nucleus body and the first planar portion non-parallel withthe second planar portion, wherein the planar portions of the nucleusbody maintain a fixed angle of correction; and wherein the nucleus bodyis composed of a non-resilient material chosen from the group consistingof: polyurethane, polymer, polyethylene, ultra-high molecular weightpolyethylene, ceramic, metal, metal alloy, titanium, titanium alloy,chrome-cobalt-molybdenum, cobalt 28 chromium molybdenum, cobalt chrome,and stainless steel.
 13. The nucleus of claim 12, wherein the first andsecond bearing surfaces are separated from one another by acircumferential wall.
 14. The nucleus of claim 13, wherein the maximumheight axis is perpendicular to the circumferential wall.
 15. Thenucleus of claim 12, wherein the second bearing surface comprises asecond curved portion, the second planar portion formed on the secondcurved portion.
 16. The nucleus of claim 12, wherein at least one of thefirst and second planar portions is sloped with respect to thecircumferential wall to provide the angle of correction.
 17. The nucleusof claim 16, wherein the angle of correction is between 3 and 6 degrees.18. The nucleus of claim 12, wherein the maximum height axis is offsettoward an anterior end of the nucleus body to provide a lordoticcorrection when the nucleus body is properly positioned between firstand second end plates attached to adjacent vertebrae.
 19. The nucleus ofclaim 12 and further including first and second end plates adapted to beattached to adjacent vertebrae, the nucleus body being positionedbetween the end plates to form a total disc replacement.
 20. A nucleusfor an artificial disc for replacing the natural disc of a human spineand being adapted to fit between and move relative to first and secondend plates that are attached to adjacent vertebrae, the nucleuscomprising: a nucleus body having a superior bearing surface and aninferior bearing surface opposite the superior bearing surface, thesuperior and inferior bearing surfaces on opposite sides of acircumferential wall, the superior and inferior bearing surfaces shapedto engage and rotationally articulate relative to cooperating bearingsurfaces formed on the first and second end plates; the superior bearingsurface having a first planar portion positioned between first andsecond curved portions of the superior bearing surface, the first planarportion extending along the sagittal plane and the first and secondcurved portions convexly curved in the sagittal plane, wherein the firstplanar portion is sloped with respect to the circumferential wall, andis configured to extend across the intersection of the coronal and thesagittal planes to provide an angle of correction, the coronal and thesagittal planes defined relative to the nucleus body such that theintersection defines a vertically extending geometric center axis of thenucleus body; the inferior bearing surface having a second planarportion positioned between third and fourth curved portions of theinferior bearing surface; and wherein the angle of correction remainsconstant irrespective of any compressive load on the nucleus.
 21. Thenucleus of claim 20, wherein first planar portion has an anterior endand a posterior end, wherein the height of anterior end is greater thanthe height of the posterior end to provide a lordotic correction. 22.The nucleus of claim 20, wherein first planar portion has an anteriorend and a posterior end, wherein the height of anterior end is lowerthan the height of the posterior end to provide a kyphotic correction.23. The nucleus of claim 20, wherein the second planar portion isnon-parallel with the first planar portion.
 24. The nucleus of claim 20,wherein the angle of correction is chosen from the group consisting of 3and 6 degrees.
 25. The nucleus of claim 20, and further including firstand second end plates adapted to be attached to adjacent vertebrae, thenucleus body being positioned between the end plates to form a totaldisc replacement.
 26. The nucleus of claim 1, wherein the correctionorientation of the at least one end plate relative to the nucleusprovides a lordotic correction.
 27. The nucleus of claim 20, wherein thenucleus body is composed of a non-resilient material chosen from thegroup consisting of: polyurethane, polymer, polyethylene, ultra-highmolecular weight polyethylene, ceramic, metal, metal alloy, titanium,titanium alloy, chrome-cobalt-molybdenum, cobalt 28 chromium molybdenum,cobalt chrome, or stainless steel.
 28. The nucleus of claim 19, whereinthe planar portions of the nucleus body maintain the fixed angle ofcorrection of the first and second endplates relative to the nucleusbody irrespective of any compressive load between the first endplate andthe second endplate.
 29. The nucleus of claim 25, wherein the angle ofcorrection remains constant irrespective of any compression of thenucleus between the first and second endplates.
 30. The nucleus of claim20, wherein the circumferential wall is located on a radially projectingflange positioned between the inferior and superior bearing surfaces.